1. Field of the Invention
The invention generally relates to systems capable of transmitting electrical power without wires.
2. Background
As used herein, the term wireless power transfer refers to a process by which electrical energy is transmitted from a power source to an electrical load without interconnecting wires. Wireless power transfer is useful for applications in which instantaneous or continuous energy transfer is needed, but for which providing a wired connection is inconvenient, hazardous, or impossible.
It has been observed that while electromagnetic radiation (such as radio waves) is excellent for transmitting information wirelessly, it is generally not suitable for transferring power wirelessly. For example, if power were transferred using omnidirectional electromagnetic waves, a vast majority of the power would end up being wasted in free space. Directed electromagnetic radiation such as lasers might be used to transfer power between a power source and a device, but this is not very practical and could even be dangerous. Such an approach would also require an uninterrupted line of sight between the power source and the device, as well as a sophisticated tracking mechanism when the device is mobile.
For the foregoing reasons, conventional systems that transfer power wirelessly are typically based on the concept of electromagnetic induction rather than electromagnetic radiation. These systems include systems based on inductive coupling and systems based on so-called “resonant inductive coupling.”
Inductive coupling refers to the transfer of energy from one circuit component to another through a shared electromagnetic field. In inductive coupling, a current running in an emitting coil induces another current in a receiving coil. The two coils are in close proximity, but do not touch.
Inductive coupling has been used in a variety of systems, including but not limited to systems that wirelessly charge a battery in a portable electronic device. In such systems, the portable electronic device is placed in close proximity to a charging station. A first induction coil in the charging station is used to create an alternating electromagnetic field, and a second induction coil in the portable electronic device derives power from the electromagnetic field and converts it back into electrical current to charge the battery. Thus, in such systems, there is no need for direct electrical contact between the battery and the charging station.
Some examples of various different types of charging systems based on the principle of inductive coupling are described in U.S. Pat. No. 3,938,018 to Dahl, entitled “Induction Charging System,” U.S. Pat. No. 4,873,677 to Sakamoto et al., entitled “Charging Apparatus for an Electronic Device,” U.S. Pat. No. 5,952,814 to Van Lerberghe, entitled “Induction Charging Apparatus and an Electronic Device,” U.S. Pat. No. 5,959,433 to Rohde, entitled “Universal Inductive Battery Charger System,” and U.S. Pat. No. 7,042,196 to Ka-Lai et al., entitled “Contact-less Power Transfer,” each of which is incorporated by reference as if fully set forth herein. Examples of some conventional devices that include batteries that may be recharged via inductive coupling include the Braun Oral B Plak Control Power Toothbrush, the Panasonic Digital Cordless Phone Solution KX-PH15AL and the Panasonic multi-head men's shavers ES70/40 series.
Another example of a technology that supports the use of inductive coupling to wirelessly transfer power is called Near Field Communication (NFC). NFC is a short-range high frequency wireless communication technology that enables the exchange of data between devices over approximately a decimeter distance. NFC is an extension of the ISO/IEC 14443 proximity-card standard that combines the interface of a smartcard and a reader into a single device. An NFC device can communicate with both existing ISO/IEC 14443 smartcards and readers, as well as with other NFC devices, and is thereby compatible with existing contactless infrastructure already in use for public transportation and payment. The air interface for NFC is described in ISO/IEC 18092/ECMA-340: Near Field Communication Interface and Protocol-1 (NFCIP-1) and ISO/IEC 21481/ECMA-352: Near Field Communication Interface and Protocol-2 (NFCIP-2), which are incorporated by reference herein.
NFC devices communicate via magnetic field induction, wherein two loop antennas are located within each other's near field, effectively forming an air-core transformer. In a passive communication mode, an initiator device provides a carrier field and a target device answers by modulating the existing field. In this mode, the target device may draw its operating power from the initiator-provided electromagnetic field.
“Resonant inductive coupling” refers to a more recently-publicized type of inductive coupling that utilizes magnetically-coupled resonators for wirelessly transferring power. In a system that uses resonant inductive coupling, a first coil attached to a sending unit generates a non-radiative magnetic field oscillating at megahertz (MHz) frequencies. The non-radiative field mediates a power exchange with a second coil attached to a receiving unit, which is specially designed to resonate with the field. The resonant nature of the process facilitates a strong interaction between the sending unit and the receiving unit, while the interaction with the rest of the environment is weak. Power that is not picked up by the receiving unit remains bound to the vicinity of the sending unit, instead of being radiated into the environment and lost.
Resonant inductive coupling is said to enable relatively efficient wireless power transfer over distances that are a few times the size of the device to be powered, therefore exceeding the performance of systems based on non-resonant inductive coupling. An example of a wireless power transfer system based on resonant inductive coupling is described in U.S. Patent Application Publication No. 2007/0222542 to Joannopoulos et al., entitled “Wireless Non-radiative Energy Transfer,” which is incorporated by reference herein.
Given the explosive growth in the use of portable electronic devices such as laptop computers, cellular telephones, and portable media devices, it is anticipated that there will be a strong demand for systems that facilitate the wireless recharging of power sources based on various types of near field inductive coupling such as those described above. Indeed, it may be deemed desirable to make such systems available in public spaces such as airports or in commercial establishments such as restaurants or hotels to allow users to recharge their portable electronic devices while away from home.
The position (i.e., the orientation and/or the location) of a portable electronic device may affect the efficiency with which the portable electronic device is recharged. Unfortunately, none of the foregoing systems based on inductive coupling or resonant inductive coupling take into consideration the position of a portable electronic device for purposes of increasing the efficiency with which the portable electronic device is recharged. For example, although NFC devices may use magnetic field induction to wirelessly transfer power and/or data, it does not appear that such NFC devices are designed to use the wirelessly transferred power to recharge a power source associated with a portable electronic device, much less to take into consideration the position of the portable electronic device for increasing efficiency of such wireless power transfer.